In recent years, a technique to manufacture a thin film transistor (hereinafter referred to as a TFT) over a substrate has made a great progress, and application to an active matrix display device has been advanced. In particular, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility to a TFT formed using a conventional amorphous semiconductor film; therefore, high-speed operation is possible with the TFT formed using the poly-crystalline semiconductor film. For this reason, it has become possible that a circuit for driving a pixel, which has been mounted by an external IC chip, is formed over the same substrate as the pixel by using a TFT.
A substrate used in a semiconductor device is expected to be a glass substrate rather than a quartz substrate in terms of cost. However, the glass substrate is inferior in heat resistance and easy to deform due to the heat. Therefore, when the TFT using the poly-crystalline semiconductor film is formed over the glass substrate, laser annealing is employed to crystallize a semiconductor film in order to prevent the glass substrate from deforming due to the heat.
Compared with another annealing method which uses radiant heat or conductive heat, the laser annealing has advantages that the processing time can be shortened drastically and that a semiconductor substrate or a semiconductor film over a substrate can be heated selectively and locally so that almost no thermal damage is given to the substrate. The laser annealing method described here indicates a technique to crystallize an amorphous layer or a damaged layer formed in a semiconductor substrate or a semiconductor film, a technique to crystallize an amorphous semiconductor film formed over a substrate, and a technique to heat (anneal) a non-single crystal semiconductor film. Further, a technique applied to planarization or modification of the surface of the semiconductor substrate or the semiconductor film is also included.
The laser annealing often employs a laser beam emitted from an excimer laser. The excimer laser has advantages of high output power and capability of repetitive irradiation at high frequency. Further, the laser beam emitted from the excimer laser has another advantage of high absorption coefficient to a silicon film, which is often used as a semiconductor film. As an irradiation method of the laser beam, the following method has high productivity and is superior industrially: a laser beam emitted from an excimer laser or the like that has high output power is shaped into a linear spot on an irradiation surface by an optical system and then the irradiation position of the laser beam is moved in a minor-axis direction of the linear laser beam relative to the irradiation surface. Currently, a technique for manufacturing a liquid crystal display by forming TFTs including a semiconductor film annealed by the above method has been widely carried out.
The laser beam emitted from the excimer laser is not a continuous wave; however, a continuous wave laser beam is also applicable. In this case, when the continuous wave laser beam (hereinafter referred to as a CW laser) is shaped into a linear spot and the irradiation position of the laser beam is moved in its minor-axis direction relatively, and a large grain crystal extended in the moving direction of laser beam is formed. When a channel forming region of a TFT is manufactured in accordance with a major-axis direction of the large grain crystal, it is possible to manufacture a TFT having higher mobility than a TFT manufactured with the excimer laser. With the TFT having high mobility, circuits such as a driver and a CPU can be driven at high speed.
The laser annealing for the semiconductor film often employs a laser beam having a wavelength in a visible range or an ultraviolet range because the absorption efficiency to the semiconductor film is high. However, the wavelength emitted from a solid-state laser medium used in a CW laser is usually from red to infrared ranges, which is low in the absorption efficiency to the semiconductor film. Therefore, a non-linear optical element is used to convert the wavelength into a harmonic having a wavelength in the visible range or the ultraviolet range, and the harmonic is used in the laser annealing. The harmonic is obtained by making the fundamental wave emitted from the laser medium enter the non-linear optical element. However, when the output power of the laser is increased, the non-linear optical element is damaged due to the non-linear optical effect such as multi-photon absorption, which may result in the breakdown of the laser oscillator.
For these reasons, the CW laser manufactured currently has a maximum output power as low as approximately 15 W because of the above-mentioned problem in the non-linear optical element. In the case of the laser annealing using the CW laser, the productivity is lower than that when using the excimer laser; therefore, further enhancement of the productivity is required. For example, when crystallization is performed by laser annealing with a linear beam spot having a size of 300 μm in the major-axis direction and 10 μm in the minor-axis direction formed by shaping a CW laser providing 10 W at 532 nm, the width of the large grain crystal obtained by one scanning is approximately 200 μm.